1. Field of the Invention
This invention relates generally to air humidifiers such as used in a central HVAC system, more particularly to a water-saving humidifier control, and specifically to an air humidifier control system based on temperature differential which maintains minimal drain water with maximal humidification output regardless of environmental conditions.
2. Description of the Prior Art
Humidifiers have become integral components in residential and commercial heating, ventilating and air-conditioning (“HVAC”) systems. Typical central HVAC systems comprise a duct system in combination with a blower or fan and controls for selective or constant circulation of air through the duct system. Heating components are utilized to provide an influx of heat upon demand or as a function of the overall HVAC system. The energy present in the air, added by the heating system, is used to evaporate water from the humidifier. Typically, the evaporator pad is wetted by introducing water at the top of the evaporator pad and allowing water to saturate the pad thoroughly. The force of gravity causes the water to flow through the evaporator pad toward a drain opening. While water is flowing through the evaporator pad, warm air is circulated either through a by-pass duct or by means of a fan or blower that is integral to the humidifier. This warm air increases the rate of evaporation of water from the evaporator pad and the humidity of the air is increased.
Typical conventional flow-through humidifiers merely supply the water to the pad at a constant flow rate as long as humidity is being called for. Generally, not all of the water evaporates, and the remainder flows through the pad and is emptied through the drain section of the typical humidifier. The drained water is wasteful and costly. A typical wetted-pad, flow-through humidifier today can have drain water to evaporation ratios exceeding five gallons of drain water to every one gallon of water evaporated into the HVAC system.
Moreover, the waste of water by the typical “constant” water-flow humidifier is generally not constant. A number of variables may aggravate the waste of water. Anything that acts to reduce the output capacity of the humidifier can likewise increase the waste of water in a constant flow system. Reductions in the surface area and/or size of the evaporator pad, degradation of the physical condition of the evaporator pad, reductions in the flow rate of air through the evaporator pad, and reductions in the temperature of the air flowing through the evaporator pad can all reduce the capacity of the humidifier and therefore increase the amount of wasted water. Except maybe for the size of the evaporator pad, all of these items are variable and dependent upon the specific conditions of the system at any specific time and may also vary with the seasons and environment.
Other variables can cause the performance of a typical flow-through humidifier to vary from application to application. The flow rate of the water may not really be constant in different locations, since it is dependent, for example, upon the water pressure supplied to the humidifier. Systems provided with higher than typical water pressure may not operate at a maximum output potential due to excessive water flowing through the evaporator pad, i.e., flooding of the pad. This condition can reduce the effectiveness of the humidifier and waste even more water. Air flow rates and temperatures are not the same in all applications, which makes humidifier performance and water waste dependent on location and application. Unless the humidifier has an integral fan or blower, the flow rate of the warm air across the evaporator pad is highly dependent on location and application. More and/or warmer air flow will result in more evaporation and, thus more humidification capacity. Some HVAC systems include filtration systems which become dirty, resulting in reduced air flow. This would result in diminished airflow and evaporation and increase the amount of water wasted. The temperature of the air flowing through the evaporator pad is also crucial to the rate of evaporation, and air temperature can depend on the HVAC installation, its environment, and other factors. The type of heat source may effect air temperature and flow rate. In gas-fired systems, gas pressure fluctuations, dirty heat exchangers, and variations in the calorific value of the gas being burned all create a variable amount of heat being delivered to the air. Variable speed blowers naturally make constant water flow humidifiers subject to variations in efficiency and waste water flow. This uncertainty makes humidifier drain waste and output ratings (typically a single number of gallons per day) somewhat unreliable, especially for predicting the amount of waste water generated by the system.
Several known methods have been employed to reduce the drain water discharged from air humidification systems. U.S. Pat. No. 6,354,572 to Menassa discloses one type of humidifier utilizing a constant time interval pulsing of the water valve to reduce the total flow rate of water through the evaporator pad so that the evaporation rate may more closely match the output capacity or potential of a given pad size. This method, while simple, may result in some water savings, but it cannot maintain an optimal output capacity while saving water concurrently. The time interval may be tuned or optimized for one specific set of HVAC system conditions (air temperature, water pressure, air flow rate, etc.), but a system like this cannot automatically adjust the flow rate of water across the pad to account for any of the conditions affecting performance mentioned above. Thus, time-based humidification systems may save water compared to a typical continuous-flow humidification system, but the humidification output and waste water results still vary for each different HVAC system condition and are not optimal.
U.S. Pat. No. 6,622,993 to Mulvaney discloses another type of humidifier which employs a wick-like evaporator pad construction and a reservoir pan equipped with a water level switch so that supply water delivery is ceased when the level switch is activated. The result is a humidifier that does not waste any water or just a negligible amount. At the beginning of operation this “drainless” humidification system may introduce a constant flow of water into the reservoir where it wicks into the pad. Upon filling the reservoir, a water level switch will close and act to cease delivery of water to the evaporator pad. As water evaporates, water “wicks” up the pad until the level switch opens and allows more water to flow across the pad. This cycle repeats until there is no longer a demand for humidification. This method ensures that no waste water will drain from the humidifier, or only a minimal amount, but it may cause premature coating of the pad with minerals requiring more frequent pad replacement. Another weakness of this method is the capacity fluctuations that will occur while the water supply is not operating and the pad is “wicking”. Over time, as the pad is coated with minerals, its ability to “wick” will begin to diminish and the output of the humidifier will decrease. Yet another weakness relates to the use of a reservoir pan. The pan allows water to stand for extended periods of time and may allow the growth of bacterial microbes that may enter the airstream on subsequent operation cycles. These drainless humidification systems may save water compared to a typical flow-through humidification system, but output capacity is wholly dependent upon the condition of the evaporator pad due to the wicking action that is required.